Android仿QQ空间顶部条背景变化效果

package com.example.administrator.myview; 
import android.content.Context; 
import android.util.AttributeSet; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.widget.AbsListView; 
import android.widget.ListView; 
/** 
 * Created by Administrator on 2018/4/3 0003. 
 * 拿到ListView滚动事件 
 * 拿到高度变化 
 * 根据高度变化，设置顶部条的背景 
 * powered by Cpf.com. 
 */ 
public class ScrollChangeHeadView extends ListView { 
  private View viewHead; 
  private View topBar; 
  public ScrollChangeHeadView(Context context,AttributeSet attrs) { 
    super(context,attrs); 
  viewHead = LayoutInflater.from(context).inflate(R.layout.scroll_change_head,null); 
    addHeaderView(viewHead);//这个addHeaderView 
  setOnScrollListener(new OnScrollListener() { 
    @Override 
    public void onScrollStateChanged(AbsListView view, int scrollState) { 
    } 
    @Override 
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) { 
      if (topBar != null) { 
        //滚动中 
        int headTop = viewHead.getTop(); 
        headTop = Math.abs(headTop); 
        //0-255 0是全透明 255是不透明 
        topBar.getBackground().setAlpha(headTop); 
      } 
    } 
  }); 
  } 
  //人为的构造一个方法，用来传值，主类获取对象实例之后，通过自定义的这个方法把对象传入第二类，然后第二类才能做事 
  public void setTopBar(View v){ 
    topBar = v; 
  } 
} 

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
  android:layout_width="match_parent" 
  android:layout_height="match_parent" 
  android:orientation="vertical"> 
  <LinearLayout 
    android:layout_width="match_parent" 
    android:layout_height="100dp" 
    android:background="#0f0c2f" 
    android:gravity="center" 
    android:orientation="vertical"> 
    <TextView 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:text="我是head" 
      android:textColor="#fff" /> 
  </LinearLayout> 
</LinearLayout> 

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
  xmlns:tools="http://schemas.android.com/tools" 
  android:layout_width="match_parent" 
  android:layout_height="match_parent" 
  android:paddingBottom="@dimen/activity_vertical_margin" 
  android:paddingLeft="@dimen/activity_horizontal_margin" 
  android:paddingRight="@dimen/activity_horizontal_margin" 
  android:paddingTop="@dimen/activity_vertical_margin" 
  tools:context="com.example.administrator.myview.MainActivity"> 
  <com.example.administrator.myview.ScrollChangeHeadView 
    android:id="@+id/schv" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent"></com.example.administrator.myview.ScrollChangeHeadView> 
  <LinearLayout 
    android:id="@+id/topBar" 
    android:layout_width="match_parent" 
    android:layout_height="45dp" 
    android:background="#12b7f5" 
    android:gravity="center" 
    android:orientation="horizontal"> 
    <TextView 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:text="QQ空间" 
      android:textColor="#fff"/> 
  </LinearLayout> 
</RelativeLayout> 

package com.example.administrator.myview; 
import android.os.Bundle; 
import android.support.v7.app.AppCompatActivity; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.BaseAdapter; 
import android.widget.LinearLayout; 
import android.widget.TextView; 
public class MainActivity extends AppCompatActivity { 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activity_main); 
      LinearLayout topBar = (LinearLayout) findViewById(R.id.topBar); 
      ScrollChangeHeadView schv = (ScrollChangeHeadView) findViewById(R.id.schv); 
      schv.setTopBar(topBar); 
      schv.setAdapter(new BaseAdapter() { 
        @Override 
        public int getCount() { 
          return 100; 
        } 
        @Override 
        public Object getItem(int position) { 
          return null; 
        } 
        @Override 
        public long getItemId(int position) { 
          return 0; 
        } 
        @Override 
        public View getView(int position, View convertView, ViewGroup parent) { 
          TextView tv = new TextView(MainActivity.this); 
          tv.setText("+"+position); 
          return tv; 
        } 
      }); 
  } 
}